The signals propagated in frequency modulated (FM) broadcasting travel in a line-of-sight path. Receivers disposed in locations without a line-of-sight path to the transmitter often receive plural signals which arrive at the receiver in an out-of-phase relationship because they follow different paths due to diffraction, refraction and/or reflection. The condition is known as multipath reception. Where plural signals arriving at the receiver are out of phase, the signals can partially or completely cancel one another and significantly degrade reception quality. A known expedient for reducing the adverse effects of signal cancellation due to out of phase arrival of the transmitted signals is to provide two antennas at spaced apart locations and/or antennas of different polarizations and to connect the antenna having the stronger signal to the receiver. This is called diversity reception; the benefit accrues because the momentary multipath disturbances may not occur simultaneously at the two antennas.
A number of diversity reception systems have heretofore been proposed. The earlier of these employed a detected, or demodulated, signal to couple the antenna receiving the stronger signal to the FM receiver. In a typical system of this character switching between antennas takes place at a relatively slow rate and is audible to the listener, especially when the program material is broadcast in the typical wide band stereo mode.
Receivers with antenna switching systems of the character just described are disclosed in U.S. Pat. Nos.: 2,729,741 issued Jan. 3, 1956, to Chapman for DIVERSITY RECEPTION SYSTEM; 2,872,568 issued February 1959 to Provaz, and 4,170,759 issued Oct. 9, 1979, to Stimple et al. for ANTENNA SAMPLING SYSTEM;
A similar switching system, which would have the same drawbacks in an FM broadcast receiver system as antenna switching can be effected by a derived audio signal, is disclosed in U.S. Pat. No. 3,476,686 issued Oct. 28, 1969, to Holt, Jr., et al.
Yet another heretofore employed system for enhancing the performance of a radio frequency receiving system requires two receivers or even a receiver for each antenna if more than two antennas are employed. This is expensive and may be impractical because of space limitations in automotive and comparable applications. Multiple receiver systems are described and disclosed in U.S. Pat. Nos.: 3,537,011 issued Oct. 27, 1970, to Escoda for ANTENNA SWITCHING ARRANGEMENT FOR CONTINUOUS SEQUENTIAL SAMPLING AND SELECTION OF BEST SIGNAL and 3,670,275 issued Jun. 13, 1972, to Kalliomaki et al. for ELECTRONIC AND AUTOMATIC SELECTOR DEVICE CONNECTED BY AN ANTENNA ARRAY FORMED BY TWO OR MORE ANTENNAS.
Still other heretofore proposed antenna selection systems, such as those described in U.S. Pat. No. 3,368,151 issued Feb. 6, 1968, to Nerwey et al. for CONTINUOUS ANTENNA SELECTION SYSTEM and in U.S. Pat. No. 4,255,816 issued Mar. 10, 1981, to Grunga et al. for RECEIVING APPARATUS HAVING A PLURALITY OF ANTENNAS are designed for navigation systems, for operation at ultra high frequencies, and for other purposes and are not compatible with frequency modulated stereo signals.
Still another disadvantage of the diversity reception systems described in the foregoing patents, as well as the system of that character described in U.S. Pat. No. 4,499,606 issued Feb. 12, 1985, to Rambo for RECEPTION ENHANCEMENT IN MOBILE FM BROADCAST RECEIVERS AND THE LIKE, is that no provision is made for solving yet another problem that arises in diversity reception receivers, especially those employed in mobile applications. This is the marked deterioration in the quality of the sound which arises as the incoming signal becomes weaker, even though that signal may be free of multipath distortion.